This invention relates to superconducting magnets in general and more particularly to a method for impregnating a superconducting magnet winding.
Magnet windings with conductors of superconductive material can be used to advantage for generating strong magnetic fields. These conductors are cooled to a temperature below the so-called critical temperature of the superconductor material used for the conductors by means of a coolant, generally by means of liquid helium. The ohmic resistance of the superconductive material then disappears almost completely. Because of the correspondingly reduced power requirement, superconductor magnets, therefore, have the advantage over conventional magnets with windings of electrically normally conducting material such as copper, that stronger magnetic fields, and thereby also higher magnetic field gradients, can be produced with them.
In order to obtain very high magnetic fields and magnetic field gradients, the effective current densities in the superconducting conductors of these magnets must be chosen correspondingly high. It may be necessary to load the superconductors to nearly the current which is critical for them. Such conductors must be specially secured against mechanical instabilities, which may, for instance, consist of conductor motion. For, if a superconducting conductor has the ability to move within the magnet winding under the action of an external force, for instance, due to Lorentz forces, it can heat up due to the friction heat connected therewith or due to the conversion of kinetic energy into heat, to such a degree that its critical temperature is exceeded and it becomes normally conducting, at least at the place of the mechanical instability.
In order to prevent such instabilities of a mechanical nature and the heating up of conductors connected therewith, the individual conductors of a superconducting magnet winding can be impregnated in a manner known per se in a vacuum with a material which is subsequently hardened and thus fixes the conductors in their position. Such a vacuum impregnation is advantageous, particularly for magnet windings of thin, very brittle or breakage prone conductors, such as the Nb.sub.3 Sn or V.sub.3 Ga conductors of what are known as "in-situ" annealed superconductor magnets. "In-situ" annealed superconductor magnets are first wound with conductors which consist of the individual components, i.e., elements, of the superconductive compound to be formed, which have not yet reacted with each other. The magnet windings made from these composite conductors are then subsequently subjected to a heat treatment, during which the superconductive compound is then produced only through diffusion of the two components of the composite conductor. Through this manufacturing technique, excessive elongation of brittle superconductors can be avoided in the winding of the magnet windings.
Two methods are generally used for vacuum impregnation. According to the method described in British Pat. No. 1,443,207, additional forms, also called molds, are necessary, in which the magnets are flooded with the impregnating medium. With this method, the development of impregnating medium layers of greater or smaller, i.e., uncontrolled, thickness at the end faces and the cylindrical surfaces of the magnet winding is unavoidable. In addition to a degradation of the superconducting magnet winding, thick layers of impregnating medium, which contract heavily upon cooling down, also, in particular, exert forces on individual conductors, for instance, in the case of the leads to the necessary contacts, which can lead to a break of individual filaments of or the entire conductor. Later removal of these excess layers is time consuming and, in addition, always accompanied by the possibility of damaging the conductor. In addition, changing the dimensions of the magnet is relatively expensive, since new molds must always be made.
In another method for vacuum impregnating the conductors of a superconducting magnet winding, such molds are, therefore, dispensed with. For these methods, special coil forms with a winding support, on which the superconducting magnet winding to be made is wound, are necessary. Such a coil form is described, for instance, in German Petty Pat. No. 75 33 199. It contains a hollow cylindrical coil form, which is joined at each of its end faces to a washer-like end flange. The two end flanges are provided with respective axial holes, through which the winding space proper is connected to an outer, annular manifold. Each manifold is made by a corresponding depression in a cover plate which is required in addition to the coil form and which rests in a vacuum tight manner against the end face of the respective end flange. After the conductors of the winding are applied on this coil form and this winding package is provided with an outer enclosure which is impervious to the impregnating medium, an impregnating medium can be fed via a corresponding feed line into the one manifold and can be fed from there, via the axial holes in the end flange, into the spaces between the individual conductors of the magnet winding. On the opposite side, the impregnating medium is then drained off in a similar manner via axial holes and the manifold connected thereto as well as a corresponding discharge line through a cover plate. Even in this known method, the development of a layer of impregnating medium at the flanges cannot be avoided. These excess layers must be removed later. While with the method that uses molds, magnet coils with contacts can also be impregnated, this is not possible with the known coil form for manufacturing superconducting magnets, since the required cover plates on the flanges of the coil form no longer leave space for contacts. Contact can, therefore, be established only after the impregnation is completed and the cover plates are removed. This, however, eliminates the use of the known coil form, for instance, for "in-situ" annealed Nb.sub.3 Sn or V.sub.3 Ga magnet windings, since in these windings, the contact leads must, in general, be already preformed and installed prior to a diffusion anneal because of the relatively small bending radii, and can practically not be changed later due to the sensitivity to bending of these conductor materials. In addition, the expense for changing the dimensions of the magnet windings is also relatively high with this method, as new cover plates are necessary.